Silica yarn for textile with high thermal resistance

ABSTRACT

The invention relates to a silica yarn and to woven or nonwoven fabrics produced from said yarn, which comprises 30 to 1500 ppm by weight of aluminum and 10 to 200 ppm by weight of titanium in oxidized form, the sum of the mass of the chemical elements different from Si and O being less than 5000 ppm by weight, the following elements being absent or present in a very small quantity: boron, sodium, calcium, potassium and lithium. The fabrics comprising this silica yarn have an excellent high-temperature withstand and thus retain their flexibility for a long time at above 600° C. They are useful especially in uses requiring good high-temperature flexibility, such as for furnace seals.

The invention relates to a silica yarn and to woven or nonwoven fabricsproduced from said yarn (or “fiber”), it being possible for this yarn tobe obtained by the process which consists in mechanically drawing amolten silica preform in a flame.

For high-temperature applications (furnace seals, furnace curtains,welding curtains, ablative protection, etc.), that is to sayapplications generally above 600° C., it is desirable for the yarns tohave the best possible stability so as to allow fabrics to be made upthat have good retention of their mechanical properties despite thenumerous heating/cooling cycles, and especially their flexibility. Thisis because a fabric that stiffens when it is subjected toheating/cooling cycles becomes brittle and does not have the samelifetime as a fabric having better retention of its flexibility while itis being used.

For this type of application, it is possible to use fabrics producedfrom washed glass. The starting point for such a material is generallyan E-glass yarn (or a yarn of suitable glass compositions that aregenerally very rich in silica), which is woven or braided and then madeto undergo a washing operation using acids (for example sulfuric acid ornitric acid) so as to lower the impurity content of the silica. Giventhe very high level of impurities in the starting glass, many impuritiesgenerally still remain after the washing. The fabric thus obtained doesnot exhibit sufficient high-temperature stability. In addition, washingthe glass leads to the formation of porosity in the silica and, beforeuse, it is generally necessary to carry out a severe heat treatment (ataround 1100° C.) so as to increase the porosity of the yarns bysintering. However, this is necessarily accompanied by an undesirableshrinkage. Moreover, the porosity of such yarns is never completelyeliminated and the fabric continues to show a certain tendency to shrinkduring its use. Because of its porosity, the washed glass has a lowdensity, of less than 2.15.

To try to overcome the drawbacks of washed glass, it may be attempted touse ultrapure silicas. However, the Applicant has also discovered thatsilicas containing a very low content of Al and of Ti have a tendency tobridge (by a mechanism similar to that of sintering phenomena). This isalso detrimental to the stability of fabrics subjected toheating/cooling cycles.

As documents of the prior art, mention may be made of EP 0 510 653 (ofthe same family as U.S. Pat. No. 5,248,637), GB 824 972, U.S. Pat. No.3,092,531, EP 0 160 232 (of the same family as U.S. Pat. No. 4,786,017).

The silica yarn according to the invention solves the above-mentionedproblems. It has an excellent thermal withstand, that is to say it hasvery good stability (very low tendency to crystallize) above 600° C., oreven above 700° C., for example at less than 1100° C. or even less than1200° C. This high temperature withstand makes it possible to usearticles produced from the yarn according to the invention for longperiods at the abovementioned temperatures, for example for at least 100hours, or even at least 1000 hours or even at least 10000 hours. Inparticular, the silica yarn according to the invention makes it possibleto produce fabrics that retain excellent flexibility (i.e. highflexibility) after having been subjected to the heat treatments thathave just been mentioned. During the heat treatment, the is flexibilityof the fabric remains similar to that which it had before the heattreatment, and it may even increase.

The silica yarn according to the invention is especially applicable formaking up fabrics such as wovens, knits, braids, nonwoven fabrics(needle-punched nonwovens, felts, fleeces, etc). The abovementionedarticles may be used without a matrix between the yarns (in the case ofwovens, these are then referred to as dry wovens) such as for furnaceseals or furnace curtains. The formation of a ceramic matrix, forexample using CVI (chemical vapor infiltration) techniques is not,however, excluded.

The invention relates more particularly to wovens from 300 g/m² to 1500g/m² (generally about 600 g/m² and about 1200 g/m²) with weaves of theplain, twill or 8 h satin or 12 h satin type (AFNOR XP B38-210 to XPB38-253 standards). The invention also relates to felts and webs whosedensity varies from 4 to 35 kg per m³ and to needle-punched fabricswhose density varies from 90 to 200 kg per m³.

The silica yarn according to the invention contains very predominantlysilica. In this kind of highly silica-rich composition, it is commonpractice:

-   -   1. to mention the amount of impurity rather than the silica        content;    -   2. to characterize the amounts of impurity by giving the        contents of the chemical elements and not the contents of oxides        (contrary to what is practised in the field of glasses very much        less rich in silica).

The present application will conform to this common practice, giving forexample the content of the chemical element Al, whereas in the glassfield the content of Al₂O₃ would have been given. In the silica yarnaccording to the invention, the amount of impurity is at most 5000 ppmby weight. The term “impurity” is understood to mean any chemicalelement different from Si and 0. This means that if the silica yarnaccording to the invention contains, for example, the chemical elementAl, the latter necessarily being present in oxidized form, the Al atomsare regarded as impurities but the oxygen atoms linked to the Al atomsare not impurities. The impurity contents can be determinedconventionally by atomic absorption spectroscopy through the measurementof a wavelength in a flame.

Aluminum and titanium, both in oxidized form, are present in the silicayarn according to the invention.

Alkali metals especially soda, Na₂O, and potash, K₂O, may be introducedinto the compositions of the silica yarn according to the invention inorder to limit the phenomena of surface diffusion of alkali metals andthus limit the sensitivity of the silica yarn to the phenomena ofsintering and bridging of the filaments between them when they areexposed to high temperatures. The composition may contain only a singlealkali metal oxide (from among Na₂O, K₂O and Li₂O) or may contain acombination of at least two alkali metal oxides.

Boron, is known in glass media as having a devitrification-retardingaction, may be present in oxidized form.

The silica yarns according to the invention are obtained from asilica-based composition comprising the following elements in oxidizedform:

aluminum: 30 to 1500 ppm by weight; titanium: 10 to 2000 ppm by weightand preferably from 10 to less than 200 ppm by weight.

In addition, the composition of the yarn is such that the followingchemical elements are absent or present in oxidized form at most asfollows:

boron: less than 600 ppm by weight; sodium: less than 100 ppm by weight;calcium: less than 100 ppm by weight; potassium: less than 100 ppm byweight; lithium: less than 100 ppm by weight.

In addition, the composition of the silica yarn according to theinvention is such that the measured quantities in ppm by weight of theelements Al, K, Li and Na (represented as ppm Al, ppm K, ppm Li and ppmNa respectively) fulfil the following condition: ppm Al>ppm K⁺ ppm Li⁺ppm Na. In the silica fiber according to the invention, the mass of theelement Al is therefore preferably greater than the sum of the mass ofthe elements K, Li and Na. More preferably, the mass of the element Alis greater than twice the sum of the mass of the elements K, Li and Na.

Moreover, preferably, any element different from Si, O. Al, Ti, B, Na,Ca, K and Li optionally present in the silica yarn according to theinvention is present at less than 100 ppm by weight. Even morepreferably, the sum of the masses of all the elements different from Si,O, Al, Ti, B, Na, Ca, K and Li is less than 100 ppm by weight.

Preferably, the silica yarn according to the invention is such that theAl content is greater than 80 ppm by weight.

Preferably, the silica yarn according to the invention is such that theAl content is less than 400 ppm by weight.

Preferably, the silica yarn according to the invention is such that theTi content is less than 30 ppm by weight.

Preferably, the silica yarn according to the invention is such that theTi content is less than 200 ppm by weight.

Preferably, the silica yarn according to the invention is such that:

-   -   the B content is less than 3 ppm by weight;    -   the Na content is less than 50 ppm by weight;    -   the Ca content is less than 60 ppm by weight;    -   the K content is less than 80 ppm by weight; and    -   the Li content is less than 10 ppm by weight.

In particular, a preferred silica composition is such that:

-   -   the Al content is between 30 and 400 ppm by weight and even more        preferably between 80 and 400 ppm by weight;    -   the Ti content is between 10 and 200 ppm by weight and even more        preferably between 30 and 200 ppm by weight;    -   the B content is less than 3 ppm by weight;    -   the Na content is less than 50 ppm by weight;    -   the Ca content is less than 60 ppm by weight;    -   the K content is less than 80 ppm by weight; and    -   the Li content is less than 10 ppm by weight;        and is such that the sum of the masses of all the elements        different from Si, O, Al, Ti, B, Na, Ca, K and Li is less than        100 ppm by weight.

For example, a silica composition particularly suitable for theinvention is such that:

-   -   the Al content is equal to about 250 ppm by weight;    -   the Ti content is equal to about 100 ppm by weight;    -   the B content is equal to about 1 ppm by weight;    -   the Na content is equal to about 20 ppm by weight;    -   the Ca content is equal to about 35 ppm by weight;    -   the K content is equal to about 50 ppm by weight;    -   the Li content is equal to about 5 ppm by weight;        the sum of all the elements different from Si, 0, Al, Ti, B, Na,        Ca, K and Li being less than 100 ppm by weight.

The compositions of the silica yarn according to the invention undergoappreciably more moderate devitrification than compositions ofneighboring fields.

The silica yarn according to the invention may be produced by fiberizingto a satisfactory yield under industrial operating conditions. The yarnaccording to the invention may thus be fiberized, like molten silicayarns or silica yarns obtained by sol-gel processes.

The yarns obtained are in the form of continuous yarns whose diametermay generally range from 5 to 300 microns, more generally from 6 to 60μm and even more generally from 6 to 15 μm.

A spinning process that can be used within the context of the presentinvention is advantageously the process used conventionally by a personskilled in the art knowledgeable in silica spinning. According to thisprocess, silica preforms of generally cylindrical cross section, thediameter generally ranging from 3 to 7 mm (more generally having adiameter of about 5 mm) are advanced into a flame capable of raising thecomposition to between 1800 and 2400° C., and the silica yarn is drawnwithin the flame. In this case, the preform has the desired compositionfor the yarn at the start. The silica feeding the process is generallyobtained from substances (or products or components or materials) thatare possibly pure (coming for example from the chemical industry) butusually natural, the latter substances often including impurities intrace amounts, these raw materials (pure or natural) being mixed, inproportions suitable for obtaining the desired composition, and thenbeing melted. The temperature of the molten silica (and therefore itsviscosity) is conventionally set by the operator so as to allowfiberizing and so as to obtain the best possible fiber quality. Thefiberizing cone must be sufficiently stable. At said cone, the materialmust be sufficiently viscous to be spinnable, but sufficiently fluid tolimit the risks of yarn breakage. The temperature of the composition isconsequently set by varying the energy and temperature of the flame. Itis also possible to vary the tensile force exerted downstream of thefiberizing unit and therefore to vary the speed of the latter. Beforethey are gathered together in!the form of yarns, the filaments aregenerally twisted with a sizing composition (conventionally chosenaccording in particular to the use of the yarns) allowing them to beprotected from abrasion and making it easier for them to handled andconverted into a fabric material, while limiting the risks of breakage.Preferably, the sizing composition contains the least possible amount ofalkali and alkaline-earth metal chemical elements, that is to say thesum of the mass of Na, K, Li and Ca represents less than 100 ppm byweight and even more preferably less than 10 ppm by weight of the sizingcomposition. The silica yarn according to the invention may be spun bythis process with a speed ranging, for example, from 10 to 300 km/hour.The silica yarn obtained by this process, which does not involve washingwith acids, has a high density, of greater than 2.15 and generallyranging from 2.15 to 2.21, and as a result has a very low tendencytoward shrinkage, which may be less than 0.5%, when it is heated to hightemperature.

It is also possible to use the washed glass technique, by starting witha glass or a silica having a much higher amount of impurities than inthe case of the previous process. In the case of a glass, the usualtechnique of spinning glass may be used, by means of bushings heated byresistance heating. After spinning, the yarns are preferably sized (forexample conventionally) and then converted into a fabric and then washedwith acids so as to end up with the desired impurity contents. In thiscase, the washing generally results in the sizing composition beingremoved, which is not a problem at this stage since the yarns havealready been converted into a fabric. In this case, it is thereforepossible to use a sizing composition having a higher alkali andalkaline-earth metal content than for the previous process. In the caseof this process, the yarn, while having the excellent stabilityassociated with its composition, may also have the drawbacks that stemfrom its residual porosity.

FIG. 1 illustrates one of the principles involved in measuring thehigh-temperature withstand of wovens for the examples that follow. Forthis test, the woven (1), after heat treatment, is bonded to a steelplate (2) over a length x. The angle α, made between the tangent (4) tothe fabric passing through the end (5) not on the steel plate and thehorizontal (3), is then measured.

EXAMPLES 1 TO 6

Silica compositions whose characteristics are given in Table 1 below arecompared.

TABLE 1 Ex. ppm by weight No. Al Ti B Na Ca K Li Density 1 240 85 0.8 1934 60 1.7 2.18 2 180 120 0.8 21 14 40 7 2.18 3 18 <1 <1 0.8 2 0.6 0.72.2 (comp) 4 635 90 3.4 560 71 6 0.03 2.1 (comp) 5 1540 2600 660 6.4 6063 0.3 2.05 (comp) 6 40000 3400 160 1738 7850 6 2 2.05 (comp)

The silica of example 4 was a silica of the sol-gel type with the brandname ENKA SILICA sold by Enka. The silica of example 3 was a silica ofthe brand name QUARTZEL sold by Saint-Gobain Quartz S.A. The washedglass silica of example 5 was of the brand name REFRASIL and was sold byHitco. The washed glass silica of example 6 was of the brand nameSILTEMP and sold by Ametek. The compositions of the silicas of examples1 and 2 were prepared by melting using natural silicas or silicasobtained from pegmatite. Yarns were produced by the conventional silicayarn spinning technique. These yarns were then conventionally sized. Forall the examples, the yarns had a diameter of between 5 and 14 μm. 600g/m² glass yarn 8 h satins were produced from the yarns. Thehigh-temperature withstand of the wovens was tested by the ASTM 1388test after exposure to 1000° C. Table 2 gives the flexural rigidityvalues G in mg.cm, G being obtained by the formula G=M.C³ in which Mrepresents the mass per unit area in mg/cm² of the woven and Crepresents the “bending” length in cm, these values being obtained after10, 100 and 1000 hours in air at 1000° C., together with the valuebefore heat treatment (duration=0). The higher the value, the more rigidthe fabric. As it is desired for the fabrics to retain good flexibility(the inverse of rigidity) during the heat treatment, rigidity valuesdose to or even slightly less than the starting values are thereforesought.

TABLE 2 Duration at 1000° C. (hours) 0 10 100 1000 Example No 1 2-3 1-21-2 1-2 2 2-3 1-2 1-2 1-2 3 2-3 5 5 5 (comp.) 4 2-3 5 5 5 (comp) 5 2-31-2 1-2 4-5 (comp.) 6 2-3 1-2 1-2 4-5 (comp.)

It may be seen that the examples according to the invention correspondto lower rigidity values, which really show that the correspondingfabrics have retained their flexibility during the heat treatment, oreven that said flexibility has slightly increased over the course ofthis heat treatment. This is favorable. In contrast, the rigidity of thefabrics of the comparative examples increased during the heat treatment.The wovens were also tested by measuring the angle to the horizontal ofa woven test piece (the principle of FIG. 1). To do this, specimens of awoven 100 mm in length by 25 mm in width were cut. These specimens werethen heated to 1000° C. for 1000 hours. They were then bonded over alength of 30 mm (x=30 mm) to a steel plate and then the angle alpha madebetween the woven and the horizontal was measured. A higher angle valuemeans a more flexible material. A fabric giving an angle value close toor even slightly higher than the initial value is therefore sought. Theresults of the angle measurements are given in table 3 below.

TABLE 3 Duration at 1000° C. (hours) 0 100 1000 Example No 1 60° 70° 65°2 60° 70° 65° 3 60° 25° 25° (comp.) 4 60° 25° 25° (comp) 5 60° 70° 45°(comp.) 6 60° 70° 45° (comp.)

These results indicate that the fabrics according to the invention haveretained a flexibility close to the initial value or even slightlygreater than the initial value during the heat treatment, which isfavorable, since the angle made between the woven and the horizontal hasincreased. In the case of the comparative examples, since the value ofthe angle has decreased this means that they have become, on thecontrary, more rigid during the heat treatment.

1. A silica yarn containing aluminum and titanium in oxidized form, thesum of the mass of the chemical elements different from Si and O beingless than 5000 ppm by weight, the aluminum and titanium contents beingthe following: aluminum: 30 to 1500 ppm by weight; titanium: from 10 toless than 200 ppm by weight; the following elements being absent orpresent in oxidized form at most as follows: boron: less than 600 ppm byweight; sodium: less than 100 ppm by weight; calcium: less than 100 ppmby weight; potassium: less than 100 ppm by weight; lithium: less than100 ppm by weight.
 2. The yarn as claimed claim 1, wherein the Alcontent is between 30 and 400 ppm by weight.
 3. The yarn as claimedclaim 2, wherein the Al content is between 80 and 400 ppm by weight. 4.The yarn as claimed in claim 3, wherein the Ti content is between 30 and200 ppm by weight.
 5. The yarn as claimed in claim 1, wherein the massof the element Al is greater than the sum of the mass of the elements K,Li and Na.
 6. The yarn as claimed in claim 1, wherein the mass of theelement Al is greater than twice the sum of the mass of the elements K,Li and Na.
 7. The yarn as claimed in claim 1, wherein any elementdifferent from Si, O, Al, Ti, B, Na, Ca, K and Li present in the silicayarn according to the invention is present at less than 100 ppm byweight.
 8. The yarn as claimed in claim 1, wherein the sum of the massesof all the elements different from Si, O, Al, Ti, B, Na, Ca, K and Li isless than 100 ppm by weight.
 9. The yarn as claimed in claim 1, wherein:the B content is less than 3 ppm by weight; the Na content is less than50 ppm by weight; the Ca content is less than 60 ppm by weight; the Kcontent is less than 80 ppm by weight; and the Li content is less than10 ppm by weight.
 10. The yarn as claimed in claim 1, wherein thediameter ranges from 5 to 300 microns.
 11. The yarn as claimed in claim10, wherein the diameter ranges from 6 to 60 μm.
 12. The yarn as claimedin claim 11, wherein the diameter ranges from 6 to 15 μm.
 13. The yarnas claimed in claim 1, wherein the density ranges from 2.15 to 2.21. 14.A fabric comprising a yarn of claim
 1. 15. The fabric as claimed inclaim 14, wherein it is a woven or a knit or a braid or a nonwovenfabric.
 16. The fabric as claimed in claim 15, wherein it is a 300 to1500 g/m² woven.
 17. The fabric as claimed in claim 1, wherein it is afurnace seal or a furnace curtain.
 18. A process comprising: heating afabric comprising the yarn claimed in claim 1 to a temperature of 600°C. or higher.
 19. The process as claimed in claim 18, wherein the fabricis in the form of a furnace seal, a furnace curtain, a welding curtain,or an ablative protectant.
 20. The process as claimed in claim 18,wherein the flexural rigidity value of the fabric does not increaseafter heating for 100 hours at 1,000° C.
 21. The process as claimed inclaim 18, wherein the flexural rigidity value of the fabric does notincrease after the fabric is heated for 1,000 hours at 1,000° C.
 22. Theprocess as claimed in claim 18, wherein the fabric is heated to atemperature of 700° C. or higher.
 23. The process as claimed in claim18, wherein the fabric is heated to a temperature of from 600 to 1,200°C.
 24. The process as claimed in claim 18, wherein the fabric is heatedto a temperature of from 600° C. to 1,100° C.
 25. The fabric as claimedin claim 14, wherein the flexural rigidity value of the fabric does notincrease after heating to 1,000° C. for 1,000 hours.
 26. The fabric asclaimed in claim 14, wherein the angle to the horizontal of a 100 mm by25 mm piece of the fabric does not decrease after heating to 1,000° C.for 1,000 hours.
 27. The fabric as claimed in claim 14, wherein theangle to the horizontal of a 100 mm by 25 mm piece of the fabricincreases after heating to 1,000° C. for 1,000 hours.